DNA polymerases are responsible for the replication and maintenance of the genome, a role that is central to accurately transmitting genetic information from generation to generation. DNA polymerases function in cells as the enzymes responsible for the synthesis of DNA. They polymerize deoxyribonucleoside triphosphates in the presence of a metal activator, such as Mg2+, in an order dictated by the DNA template or polynucleotide template that is copied. In vivo, DNA polymerases participate in a spectrum of DNA synthetic processes including DNA replication, DNA repair, recombination, and gene amplification. During each DNA synthetic process, the DNA template is copied once or at most a few times to produce identical replicas. In contrast, in vitro, DNA replication can be repeated many times such as, for example, during polymerase chain reaction (see, e.g., U.S. Pat. No. 4,683,202 to Mullis).
In the initial studies with polymerase chain reaction (PCR), the DNA polymerase was added at the start of each round of DNA replication (see U.S. Pat. No. 4,683,202, supra). Subsequently, it was determined that thermostable DNA polymerases could be obtained from bacteria that grow at elevated temperatures, and that these enzymes need to be added only once (see U.S. Pat. No. 4,889,818 to Gelfand and U.S. Pat. No. 4,965,188 to Mullis). At the elevated temperatures used during PCR, these enzymes are not irreversibly inactivated. As a result, one can carry out repetitive cycles of polymerase chain reactions without adding fresh enzymes at the start of each synthetic addition process. DNA polymerases, particularly thermostable polymerases, are the key to a large number of techniques in recombinant DNA studies and in medical diagnosis of disease. For diagnostic applications in particular, a target nucleic acid sequence may be only a small portion of the DNA or RNA in question, so it may be difficult to detect the presence of a target nucleic acid sequence without amplification.
The overall folding pattern of DNA polymerases resembles the human right hand and contains three distinct subdomains of palm, fingers, and thumb. (See Beese et al., Science 260:352-355, 1993); Patel et al., Biochemistry 34:5351-5363, 1995). While the structure of the fingers and thumb subdomains vary greatly between polymerases that differ in size and in cellular functions, the catalytic palm subdomains are all superimposable. For example, motif A, which interacts with the incoming dNTP and stabilizes the transition state during chemical catalysis, is superimposable with a mean deviation of about one A amongst mammalian pol α and prokaryotic pol I family DNA polymerases (Wang et al., Cell 89:1087-1099, 1997). Motif A begins structurally at an antiparallel β-strand containing predominantly hydrophobic residues and continues to an α-helix. The primary amino acid sequence of DNA polymerase active sites is exceptionally conserved. In the case of motif A, for example, the sequence DYSQIELR (SEQ ID NO:22) is retained in polymerases from organisms separated by many millions years of evolution, including, e.g., Thermus aquaticus, Chlamydia trachomatis, and Escherichia coli. 
In addition to being well-conserved, the active site of DNA polymerases has also been shown to be relatively mutable, capable of accommodating certain amino acid substitutions without reducing DNA polymerase activity significantly. (See, e.g., U.S. Pat. No. 6,602,695 to Patel et al.). Such mutant DNA polymerases can offer various selective advantages in, e.g., diagnostic and research applications comprising nucleic acid synthesis reactions.
There are at least two steps in the enzymatic process of DNA polymerization; 1) the incorporation of the incoming nucleotide and 2) the extension of the newly incorporated nucleotide. The overall faithfulness or “fidelity” of the DNA polymerase is generally thought of as a conglomerate of these two enzymatic activities, but the steps are distinct. A DNA polymerase may misincorporate the incoming nucleotide, but if it is not efficiently extended the extension rate will be severely decreased and overall product formation would be minimal. Alternatively, it is possible to have a DNA polymerase misincorporate the incoming nucleotide and readily misextend the newly formed mismatch. In this case, the overall extension rate would be high, but the overall fidelity would be low. An example of this type of enzyme would be ES112 DNA polymerase (E683R Z05 DNA polymerase; see U.S. Pat. No. 7,179,590, entitled “High temperature reverse transcription using mutant DNA polymerases” filed Mar. 30, 2001 by Smith et al., which is incorporated by reference) when using Mn2+ as the divalent metal ion activator. The enzyme has a very high efficiency because unlike typical DNA polymerases that tend to hesitate/stall when a mismatch is encountered, the ES112 DNA polymerase readily extends the mismatch. The phenotype displayed in ES112 is more pronounced during the RT step, presumably because of structural effects of the RNA/DNA heteroduplex vs. the DNA/DNA homoduplex. A second example would be if the DNA polymerase does not readily misincorporate (may be even less likely to misincorporate), but does have increased capacity to misextend a mismatch. In this case, the fidelity is not significantly altered for the overall product. In general, this type of enzyme is more favorable for extension reactions than the characteristics of ES112 in Mn2+ because the fidelity of the product is improved. However this attribute can be utilized to allow the misextension of a mismatched oligonucleotide primer such as when an oligonucleotide primer of a single sequence is hybridized to a target that has sequence heterogeneity (e.g., viral targets), but the normal or lower misincorporation rate allows for completion of DNA synthesis beyond the original oligonucleotide primer. An example of this type of DNA polymerase is Z05 D580G DNA polymerase. (see U.S. Patent Publication No. 2009/0148891 entitled “DNA Polymerases and Related Methods” filed Oct. 17, 2007 by Bauer et. al., which is incorporated by reference). This type of activity is referred to as “mismatch tolerant” because it is more tolerant to mismatches in the oligonucleotide primer. While the examples above have discussed primer extension type reactions, the activity can be more significant in reactions such as RT-PCR and PCR where primer extension is reoccurring frequently. Data suggests that while enzymes such as Z05 D580G are more “tolerant” to mismatches, they also have enhanced ability to extend oligonucleotide primers containing modified bases (eg., t-butyl benzyl modified bases) or in the presence of DNA binding dyes such as SYBR Green I (see U.S. Patent Publication No. 2009/028053 entitled “Improved DNA Polymerases and Related Methods” filed Apr. 16, 2009 by Bauer et al., which is incorporated by reference).
Reverse transcription polymerase chain reaction (RT-PCR) is a technique used in many applications to detect/and or quantify RNA targets by amplification. In order to amplify RNA targets by PCR, it is necessary to first reverse transcribe the RNA template into cDNA. Typically, RT-PCR assays rely on a non-thermostable reverse transcriptase (RNA dependent DNA polymerase), derived from a mesophilic organism, for the initial cDNA synthesis step (RT). An additional thermostable DNA polymerase is required for amplification of cDNA to tolerate elevated temperatures required for nucleic acid denaturation in PCR. There are several potential benefits of using thermoactive or thermostable DNA polymerases engineered to perform more efficient reverse transcription for RT-PCR assays. Increased reverse transcriptase activity coupled with the ability to use higher reverse transcription incubation temperatures, which allow for relaxing of RNA template secondary structure, can result in overall higher cDNA synthesis efficiency and assay sensitivity. Higher temperature incubation could also increase specificity by reducing false priming in the reverse transcription step. Enzymes with improved reverse transcription efficiency can simplify assay design by allowing for reduced RT incubation times and/or enzyme concentration. When using dUTP and UNG, nonspecific extension products containing dUMP that are formed during nonstringent set-up conditions are degraded by UNG and cannot be utilized either as primers or as templates. When using a non—thermostable reverse transcriptase (RNA dependent DNA polymerase) derived from a mesophilic organism, it is not possible to utilize the dUTP and UNG methodologies. (Myers, T. W. et al., Amplification of RNA: High Temperature Reverse Transcription and DNA Amplification with Thermus thermophilus DNA Polymerase, in PCR Strategies, Innis, M. A., Gelfand, D. H., and Sninsky, J. J., Eds., Academic Press, San Diego, Calif., 58-68, (1995)). However, the use of a thermoactive or thermostable DNA polymerase of the invention for the reverse transcription step enables the reaction to be completely compatible with the utilization of the dUTP/uracil N-glycosylase (UNG) carry-over prevention system (Longo et al., Use of Uracil DNA Glycosylase to Control Carry-over Contamination in Polymerase Chain Reactions. Gene 93:125-128, (1990). In addition to providing carry-over contamination control, the use of dUTP and UNG provides a “hot-start” to reduce nonspecific amplification (Innis and Gelfand 1999).